This invention relates generally to non-destructive inspection of structures and, more particularly, to apparatus for analyzing nodal patterns induced in the outer skin of monocoque and/or semi-monocoque structures. More specifically, this invention relates to apparatus for analyzing holographic records of nodal patterns obtained by techniques described in copending patent application Ser. No. 08/011,991, filed Feb. 1, 1993 by applicant Webster entitled "Method and Apparatus for Nondestructive Inspection Utilizing Phase Integration and Recording of Induced Vibrating Nodal Patterns", the disclosure of which is hereby incorporated herein by reference.
Briefly, the method described in the aforementioned application includes the steps of mechanically exciting a vibrating nodal pattern in the outer skin of a monocoque structure, panels of which may freely vibrate at one or more resonant frequencies; scanning the frequency of excitation through a predetermined spectrum of frequencies which include such resonant frequencies and locking onto a resonant frequency which is uniquely characteristic of a certain phase relationship of the anti-nodes of the nodal pattern; varying the amplitude of excitation of the structure until the anti-nodes are optimized and when such optimization is reached, recording two time-displaced overlying holograms of the outer skin in synchronism with maximum plus and minus displacement, respectively, of an anti-node. Interference patterns are generated at a separation of approximately one-half wavelength of light, contouring the anti-nodal patterns; this fringe map of contour patterns provides information relating to the integrity of the structure and enables diagnosis of faults, both surface and subsurface.
The present invention is directed to a computer-based system for analyzing nodal patterns induced in the surface of a structure being inspected, either by analyzing a record obtained by holographic interferometry in the manner described in the aforementioned patent application, or by directly scanning, or digitally interrogating point-by-point, the vibrating surface of the structure with a beam of quasi-coherent radiant energy and, utilizing the Doppler effect, reading the out-of-plane displacement of the surface to produce a visual contour map of the surface for capture and analysis by computer. Alternatively, a computer may be programmed to directly capture the nodal pattern and either provide a direct readout of the location and/or identity of structural faults or the nodal pattern is captured directly for later processing.
Accordingly, a general object of the present invention is to improve the speed and accuracy of analysis of the results produced by vibrational techniques for detection of flaws in monocoque and semi-monocoque structures.
Another object of the invention is to provide a system for analyzing a holographic record of nodal patterns induced in the surface of a structure being inspected for diagnosing faults and to provide information relating to the integrity of the structure.
Another object of the invention is to provide apparatus for identifying structural faults in a structure by analysis of images of nodal patterns induced in such structure.
Still another object of the invention is to provide apparatus for identifying structural faults in a structure by quasi-real-time analysis of nodal patterns induced in such structure.